Structure and petrology of the Tregonning—Godolphin granite, Cornwall

Abstract

Field evidence indicates that the plutonic history of the complex began with the emplacement of fine-porphyritic biotite granite (Godolphin granite and early granite porphyries). This was followed by the passive emplacement, by cauldron subsidence and block stoping, of the lithium-rich Tregonning granite. Differentiation of this mass in situ gave rise to the rocks forming the Roof Zone and the granite sheets of Tremearne, composed of leucogranites and aplitepegmatite bodies. Late granite porphyries (‘elvans’) cut the Tregonning granite. Greisenisation and kaolinisation affected all rock types and completed the plutonic history. Statistical considerations lead to the conclusion that the chemical data can be considered to be distributed normally for the purpose of correlation and factor analysis. Two important associations are Ti, Fe, Mg, K and Na, Li, Mn. The latter increase and the former decrease with time in the sequence biotite granite, lithium mica granite (Tregonning), leucogranite and aplite. Lithium-mica granite is believed to have been derived from biotite granite by LiAl ⇌ 2(Mg, Fe) and alkali-ion exchange reactions together with the deanorthitisation of plagioclase feldspar in the presence of abundant volatiles rich in fluorine. Such transformations could result in increased mobilisation through local melting as compositions approach a minimum melting point or eutectic. Some repetition of the exchange processes and an increase in F-ion activity in the fluid phase is invoked to explain the concentration of leucogranites at the roof and the evidence for several generations of aplite-pegmatite bodies. © 1975, The Geologists' Association. All rights reserved.